1. Field of the Invention
This invention related to a process and apparatus for filtering a substance flowing through a passage by introducing a filter across the passage, and, more particularly, to an improved method and apparatus for the filtering of heat-softened plastics materials.
2. Description of the Related Art
It is often necessary to purify molten resins, having a viscosity of at least 100 cp, during processing when impurities are present in unacceptable concentrations. This is usually accomplished by straining the thermally softened plastics through a filter medium, usually downstream of an extruder. Such straining can take place in the same barrel used for the extrusion process or in a device completely independent of the extruder. The device of this invention can also be used in a process without regard to whether the processing also includes extrusion.
In time, the filter medium becomes clogged and must be replaced. Opening up the extruder in order to replace the filter interrupts the production and considerably inconveniences the operators who risk coming in contact with the potentially hot and sticky melt in the process of replacing or cleaning the clogged filter. Whether the clogged filter is replaced or cleaned depends on the filter material and the nature of the material being filtered.
The filter medium is usually configured in the form of a screen. A large variety of screen melt filter devices is available on the market or described in the literature. The devices can roughly be classified into two broad categories--screen changing devices and continuous devices.
It is well known that some of the dangers and inefficiencies inherent in the changing of a filter screen can be alleviated by employing two or more screens, each mounted on separate mountings, so that when one screen becomes clogged, a clean screen can quickly be inserted to replace the clogged screen. The switchover is achieved either by incorporating the several screens in a slide or similar device which can be periodically reciprocated or rotated so as to allow a new screen to replace a clogged one, or by redirecting the flow from a clogged filter screen to a clean screen by means of a valve or similar device. A number of specific screen changing devices and mechanisms are described in U.S. Pat. No. 4,842,750, the disclosure of which is hereby incorporated by reference.
A problem often associated with screen changing devices is the interruption of the melt flow during the screen changing. This interruption has a number of undesirable effects. First, the interruption of what would otherwise be a continuous process turns the process into a batch process, with all the attendant disadvantages of batch processes, such as lack of uniformity of product and conditions, dynamics in the system, increased requirement of operator attention and involvement, and reduced reliability. In addition, the operators are forced to handle the clogged filters which are coated with hot, viscous, and sticky melts. Since the filter is taken off-line during the screen changes, a larger filter area, typically in the form of multiple filter screens, must be used in order to accommodate the melt throughflow. Furthermore, an inferior product is produced. Since the quality and dimensional stability of the extrudate are directly related to the pressure variation at the screen, it is usually desired that the pressure at the screen be constant and not vary cyclically with the changing of the screens.
Many screen changing devices call for monitoring the pressure drop across the screen and effectuate the changing of the screens when the pressure drop reaches a certain preset value. This mode of operation results in a monotonically increasing pressure drop across the filter as the filter becomes progressively more clogged, followed by a sudden drop in differential pressure as the clogged screen is removed and a clean screen is put in place. The pressure then again proceeds to increase monotonically as the new screen becomes more clogged, and so on. The effect of this varying pressure differential on the quality and dimensional stability of the extrudate is usually detrimental.
In order to avoid many of the problems associated with screen changers, devices have been proposed, and in some cases commercialized, which replace individual discrete screens with screens in the form of continuous strips which can be advanced when a section becomes dirty. These constantly operating filters have a number of advantages over screen changing devices. Among these are the following: 1) the continuous filters, because they allow for essentially constant filter throughflow, require a smaller screen area; 2) the continuous filters are more conducive to small and compact design; 3) the continuous filters produce constant conditions in the melt channel; and, 4) the continuous filters allow for uniform throughflow.
A number of specific continuous filter devices and mechanisms are described in U.S. Pat. No. 4,849,113, the disclosure of which is hereby incorporated by reference.
Unfortunately, the use of long strips of filter media raises sealing problems, since the mechanism for holding a filter medium in place must periodically be released so as to allow the filter strip to be advanced. Such a release allows the melt, which is under high pressure, to escape to the outside. This results in loss of material and in a messy and potentially dangerous operating environment.
A number of devices incorporating a long strip of filter medium are known. Each strives to solve the sealing problem. U.S. Pat. No. 2,867,324 discloses a method of sealing a continuous filter which includes inflatable sealing tubes. In the inflated condition, these tubes bear under pressure on both sides of the filter medium. When deflated, the sealing tubes allow the filter medium to be advanced a predetermined amount. The tubes are then reinflated to fix the filter medium in place. The operation is intermittent and the design is not likely to be reliable in commercial operations. It is perhaps for this reason that such filters have yet to appear on the market.
Another way of handling the sealing problem is displayed by the various Kalman filters. U.S. Pat. Nos. 3,471,017; 3,645,399; 3,856,674; 3,940,335; and 3,856,680, disclose variations on a continuous filter travelling from a feed spool to a takeup spool. The sealing of the filter medium is accomplished primarily by freezing some of the polymer at the inlet and outlet of the media into the filter housing. The frozen material around the filter media keeps the melt from escaping to the outside. When it is desired to advance the filter medium, the ports are heated to partially melt the seal and to allow the filter to be advanced toward the takeup spool.
Although certain Kalman filters are available commercially, they have various limitations. The reliance on the plugs of solidified material to seal the filter has not been justified, especially for the lower viscosity melts such as epoxy resins. The Kalman filters work best for thermoplastic materials having a wide melting temperature range. Typical epoxy resins are much more uniform and tend to display a relatively narrow range of freezing points.
Operation of the Kalman filter is difficult since, in order to insure a seal, the plugs must be cooled sufficiently to form a relatively solid mass. This mass tends to bind to the inlet and outlet ports, making filter advancement difficult. If the plugs are not completely solidified, leakage of melt material occurs. In addition to requiring very precise and careful control, it has been found that many materials cannot be handled by such filters. For example, materials which are lower in viscosity will not solidify quickly enough or at high enough temperatures to make the operation of the filter efficient.
In addition, such filters do not function properly where the material, such as epoxies, to be filtered lacks self lubricating properties since such materials would adhere strongly to the filter body and cannot easily be separated from the filter body to allow filter movement. Also, such devices require high strength filter media such as metal screen and are unlikely to function properly with certain desirable synthetic aramid fiber filter media such as NOMEX or KEVLAR, which are registered trademarks of DuPont. These devices are also unlikely to function well with other flexible woven matted fabric filter media such as cloth, nylon, and rayon.
U.S. Pat. No 4,842,750 discloses a continuous filter for the filtering of viscous material which discretely feeds a filter medium from a feed spool to a takeup spool based on the differential pressure across the filter. When the filter medium is to be moved, a mechanism is activated which momentarily increases the volume just upstream of the filter. This reduces the pressure on the filter and enables the filter medium to be moved, thereby exposing the new filter medium and removing the clogged filter. Operation of this device is discontinuous.
U.S. Pat. No. 4,849,113 to Hills discloses a polymer filter having two filtering chambers, each with its own associated feed spool and takeup spool. The device is activated by differential pressure across the filters. A three way valve upstream of the filter is intermittently positioned so as to reduce the polymer flow into one of the two chambers and thereby reduce the pressure on the filter associated with that filter. This allows the filter to be pulled through the housing, exposing new filter area to the polymer.
This device, like the other continuous devices described above, is not capable of truly continuous operation. While the filter medium in each case is a long strip which is, in theory, capable of continuous movement, in practice, each of the above devices uses a set of discrete movements, usually controlled by filter pressure drop, to move the filter medium incrementally through the filtering device.
There has long been a need for a continuous melt filter capable of: operating on a truly continuous basis, and thus capable of maintaining fixed conditions in the melt being filtered; filtering melts which are not self-lubricating; filtering melts without excessive leakage of melt material to the outside; and filtering less viscous melts such as epoxy resins.